Each reverse osmosis membrane element is made from a reverse osmosis membrane flat sheet. There are several manufacturers of RO membrane flat sheets in the world. The largest are DOW Chemical, GE, Toray, and Hydranautics. Each brand offers RO membrane flat sheets with unique specifications. The chemical composition of each RO membrane is protected by patents and remains a well-guarded secret. Reverse osmosis membrane flat sheets are usually classified by porosity, flow rate, and salt rejection under given feed water concentration, temperature, and pressure. High-quality RO membrane housings are also important in the production of the RO membrane.